This invention relates to repairing tubing of a heat exchanger and, more particularly, to a method for repairing the input ends of the heat exchanger tubes.
Heat exchangers such as surface condensers used in steam power plants to condense steam turbine exhaust contain a large number of relatively thin walled, small diameter tubes through which cooling water is passed. There may be thousands of such tubes of differing lengths, for example 30 to 50 feet long, which extend through a generally cylindrical condenser housing between water boxes at either end. The exhaust steam from the turbine low pressure stages is condensed on the cold surfaces of the tubes and gathers in a hot well from which it is returned to the boiler.
These condensers and other shell and tube heat exchangers primarily fail due to inlet end corrosion or erosion and tube end cracking. Experience shows that the failure rate of tubes due to inlet end related problems amount to between 80 and 90% of all such failures. This sort of failure mechanism attacks all or most of the tubes to more or less the same degree. The attack finally results in massive or catastrophic tube failures necessitating costly and time consuming relining or retubing. In power stations this can mean shutting down the entire plant for up to three months. In ships, particularly combat vessels, retubing may require cutting through the ship's hull and deck for equipment removal to permit access to perform the retubing function.
Aside from the total tube replacement or total relining of the damaged tube, numerous corrective measures have been tried with varying degrees of success. For example, inserts for protecting tubes from erosion and corrosion damage are disclosed in U.S. Pat. Nos. 1,937,600; 2,157,107; 2,262,042; 2,445,273; and 2,620,830. All these inserts disclosed in the aforesaid patents rely on a press fit, adhesive, or fiber type rings to establish a seal between the insert and the tube. In order to restore the heat exchanger to design conditions, the tube to tube sheet joint must be restored. None of the aforesaid patents take this joint integrity into account. In order to restore the joint strength, metal must replaced where it has been eroded or corroded away.
In U.S. Pat. No. 3,781,966 there is disclosed the use of two metallic sleeves with the first sleeve being inserted and expanded into engagement with the tube. Although the sleeve fractures in the region where it expands the most, this is overcome by inserting a second metallic sleeve into the first which is again expanded into engagement with the fractured first sleeve. The second sleeve has substantially the same thickness and outside diameter as the first sleeve but it is a shorter length since it can only be inserted part way into the first sleeve. This procedure is time consuming and provides no assurance that the second sleeve will not be fractured like the first, requiring a complete replacement of the tube and defeating the purpose of the repair operation. Since the first sleeve is fractured, joint strength integrity is compromised. In addition, the interior diameter of the tubing is uneven and may restrict the fluid flow therein.